The present invention generally relates to a method and apparatus for the purification and disinfection of water. More specifically, the present invention relates to an apparatus and method of use of a semiconductor material for the photocatalytic degradation of organic and inorganic pollutants and microorganisms in water and ultrapure water1. The present invention is an apparatus and method incorporating a rigid, open cell, three dimensionally reticulated, fluid permeable, photocatalytic semiconductor unit.
1 Ultrapure water as used herein refers to water that is pretreated by methods know to those skilled in the art to remove suspended and dissolved inorganic and organic mater from municipal, well water and any other water source. 
Heterogeneous photocatalysis is the general term that describes the technical approach, [Mills, A.; Le Hunte, S.; xe2x80x9cAn Overview of Semiconductor Photocatalysis,xe2x80x9d J. PhotoChem. and PhotoBio. A: Chemistry 108 (1997) 1-35] and [Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.; xe2x80x9cEnvironmental Applications of Semiconductor Photocatalysis,xe2x80x9d Chem Rev 1995, 95, 69-96]. The specific process is properly described as semiconductor-sensitized photomineralization of organics by oxygen. It may be summarized as:
xe2x80x83Organic pollutant+O2CO2+H2O+mineral acid
hv greater than Ebg
where hv represents the energy of a photon and Ebg is the bandgap energy separating electrons in the valence band of the semiconductor from those in its conduction band.
The process is driven by photons having more energy than the bandgap of the semiconductor they irradiate. Each such photon absorbed by the semiconductor will promote an electron from the valence band producing a conduction band electron (exe2x88x92) and a valence band hole (h+). When the resultant electron-hole pair migrates to the semiconductor/solution interface, oxidation-reduction processes are initiated. These include:
Holes:
Acidic or neutral solutions: H2O+h+OH.+H+
Alkaline solutions: OHxe2x80x94+h+OH.
Electrons:
Uncertain reaction pathway resulting in the reduction of oxygen to various reactive species including
O., O2., O2H., HO2xe2x80x94, H2O2 and OH.
Of particular importance is the formation of OH., the hydroxyl radical. The hydroxyl radical is an extremely potent oxidizing agent (redox potential of +2.8 V), capable of oxidizing almost all organic compounds. By comparison, the redox potentials for the more conventional oxidants chlorine and ozone are +1.36 and +2.07 V, respectively. Hydroxyl radicals also kill and breakdown microorganisms. The reactive species created by the reduction of oxygen will also oxidize organic compounds. All active species are created from water, and decay back to water. Light is the only reagent required.
Semiconductor photocatalysts that have been demonstrated for the destruction of organic contaminants in fluid media include but are not limited to: TiO2, ZnO, CaTiO3, SnO2, MoO3, Fe2O3, and WO3. TiO2 is the most widely investigated because it is chemically stable, has a suitable bandgap structure for UV/Visible photoactivation, and is relatively inexpensive.
TiO2 exists in two principal crystalline forms: rutile and anatase. The rutile form of TiO2 is widely used as a pigment and can be found in almost anything whitexe2x80x94paint, paper, textiles, inks, plastics and cosmetics. Anatase, the low temperature form (stable below xcx9c600xc2x0 C.) is the most photoactive form. Nanoscale (5-50 nm) anatase particles with very high surface areas (50-500 m2/gm) show high photoactivity when irradiated with UV light ( less than 390 nm) in the presence of water.
The deposition of a transition metal (e.g., platinum, palladium, silver) on the surface of the anatase increases the photocatalytic activity by approximately a factor of two. A variety of methods improve the quantum efficiency of TiO2 by doping with various metals to extend the spectral response into the more efficient visible light wavelengths, [Borgarello, E. et al. xe2x80x9cVisible Light Induced Water Cleavage in Colloidal Solutions of Chromium-Doped TiO2 Particles,xe2x80x9d J. Am. Chem. Soc. 1982, 104, 2996-3002] or to increase the minority carrier diffusion length, [Augustynski, J.; Hinden, J. Stalder, C.; J. Electrochem. Soc. 1977, 124, 1063] or achieve efficient charge separation to increase carrier lifetimes, Vogel, R.; Hoyer, P; Weller, H.; xe2x80x9cQuantum-Sized PbS, CdS, Ag2S, Sb2S3 and Bi2S3 Particles as Sensitizers for Various Nanoporous Wide-Bandgap Semiconductors,xe2x80x9d J. Phys. Chem. 1994, 98, 3181-3188].
Most of the early research on semiconductor photocatalysis concerned methods using titanium dioxide (TiO2) slurries or TiO2 wash coatings onto or inside a glass tube and the photodegradation of organic compounds and their intermediates in water. These methods of using TiO2 have limitations for commercial applications. For example, although TiO2 slurry has tremendous surface area and has acceptable quantum yields, there are serious limitations to the removal of the TiO2 particles from the purified water. While wash coating TiO2 onto glass avoids the removal limitations of the slurry approach, it has its own problems in that insufficient surface area is presented for effective destruction of organics within a reasonable time period. Additionally, the wash coat is not firmly attached to the glass resulting in a loss of TiO2 to the water stream and a concomitant reduction in photocatalytic activity.
Kraeutler and Bard made a photocatalytic reactor of water slurry of suspended TiO2 powder, in the anatase crystalline form, and studied the decomposition of saturated carboxylic acid, [J. ACS 100 (1978) 5985-5992]. Other researchers used UV-illuminated slurries of TiO2 for the photocatalyzed degradation kinetics of organic pollutants in water.
Mathews created a thin film reactor by wash coating TiO2, (Degussa P25(trademark)), particles to the inside of a 7 millimeter long borosilicate glass tube wound into a 65-turn spiral. The reactor was illuminated with a 20 watt, black light UV fluorescent tube. He monitored the destruction of: salicylic acid, phenol, 2-chlorophenol, 4-chlorophenol, benzoic acid, 2-naphthol, naphthalene, and florescin in water, [J. Physical Chemistry 91 (1987) 3328-3333].
As an improvement over the prior art approaches, U.S. Pat. No. 4,892,712 to Robertson et al. disclosed the attachment by the sol-gel process of anatase TiO2 to a fiberglass mesh substrate. This mesh was wrapped around a light source contained within a quartz glass cylinder and emitting UV radiation in a wavelength range of 340 to 350 nanometers (nm). The entire structure was placed within a stainless steel cylinder containing fluid inlet and outlet ports thereby creating a reactor. Polluted water was passed through this reactor for purification. Unlike the present invention, Robertson""s mesh is not rigid, open cell, three dimensionally reticulated and lacks permanent bonding of the semiconductor to the mesh.
Professor I. R. Bellobono prepared photocatalytic membranes immobilizing 23% of Titanium Dioxide (Degussa P-25). Controlled amounts of appropriate monomers and polymers, containing the semiconductor to be immobilized and photoinitiated by a proprietary photocatalytic system was photografted onto a non-woven polyester tissue. The final porosity of the photosynthesized membrane was regulated at 2.5-4.0 microns. He trade named this membrane xe2x80x9cPhotopermxe2x80x9d(trademark). A fluid containment structure surrounded the membrane creating a reactor. The reactor volume occupied by the fluid was 2.5 liters (l) and the membrane surface area was 250 square centimeters (cm2). The reactor was illuminated with a cylindrical high-pressure mercury arc lamp at a power of 2 kilowatts (kW) and at a wavelength of 254 nm. Water flowed into the center of the reactor and moved out tangential to the lamp through the membrane. This system was used to destroy phenol in water, [xe2x80x9cEffective Membrane Processes. New Perspectivesxe2x80x9d (R. Paterson, ed.) BHR, Mech. Eng. Publ., London (1993), pg 257-274]. The process was patented in Italy in 1995, Italian Pat. No. IT1252586. Unlike the present invention, Bellobono""s apparatus is not inert, not open cell, not three dimensionally reticulated and not durable.
Cittenden, et al. discloses a method and apparatus for destroying organic compounds in fluids [The 1995 American Society of Mechanical Engineers (ASME) International Solar Energy Conference, Maui, Hawaii, USA]. TiO2 was attached by wash coating to a 35xc3x9760-mesh silica gel substrate. The substrate was placed within a plastic tube that allowed the penetration of UV light. Organic pollutants in a water stream passed axially through the tube. Natural light and/or artificial UV light oxidize the investigated organic pollutants. Unlike the present invention, Cittenden""s invention is not open cell, not three dimensionally reticulated, not durable, and has very limited fluid permeability.
Anderson discloses a method to make ceramic titanium membranes by the sol-gel process. [J. Membrane Science 30 (1988) 243-258]. These membranes are porous and transparent to UV illumination. They are made from a titanium alkoxide and then fired to form the anatase crystalline structure. Unlike the present invention, Anderson""s invention is not open cell, not three dimensionally reticulated, not durable, and has very limited fluid permeability.
Thus, while attempts were made in the prior art to enhance quantum yields by increasing semiconductor surface area and improving UV light penetration, serious limitations remain to the commercial development of an efficient, durable photocatalytic purification apparatus and method for its use. In Robertson, in addition to the severe limitations already above noted, the flexible strands of fiberglass precluded the permanent attachment of TiO2 because, as water passed by, the fiberglass strands bent and flexed releasing TiO2 particles, particularly at high fluid flow rates. For Bellobono, in addition to all the limitations also above noted, the photocatalytic process gradually oxidized the organic membrane reducing its activity over time. In addition to all the limitations also above noted, Cittenden""s TiO2 sloughed-off because it was wash coated to the silica gel substrate. In addition, the void space between silica particles was so small that flow through the system was restricted making the structure unsuitable for commercial applications. In Andersen""s membrane, in addition to the limitations above noted, limitations on the structural integrity of these membranes exist particularly at high fluid velocities needed for efficient industrial applications.
The object of the present invention is to substantially improve upon the prior art to produce an effective, quantum efficient, durable, economic, commercial apparatus for the rapid photocatalytic purification and disinfection of water and ultrapure water. At the present time in the semiconductor processing industry, current technology struggles to achieve 2 parts-per-billion (ppb) in Total Organic Carbon (TOC). This represents a limit on the industry""s ability to achieve further improvements in the chip density and speed. The present invention, which achieves 500 parts-per-trillion (ppt) in TOC, or better, represents a breakthrough for both the water purification and semiconductor industry. The invention also has profound implications for other water purification systems, including those related to environmental cleanup.
The apparatus of the present invention involves a reactor apparatus and a method for its use for photopromoted, catalyzed degradation of compounds in a fluid stream. The effectiveness of the process is determined in part by the mass transfer efficiency, which is the rate at which the contaminant is transported from the fluid stream to the photocatalytic surface where it can be destroyed. Mass transfer is greatly aided by proximity. Thus, it is desirable, to the greatest degree possible, to have the catalyst uniformly distributed in the volume of water to be treated, such that a contaminant is never far from a catalyst surface.
Another consideration is the uniform illumination of the catalyst within the volume of water to be treated. Since the catalyst itself absorbs the light, its concentration in the volume should be limited to allow sufficient penetration of the activating photons. In addition, the support structure should not block illumination of the volume of water to be treated. Thus, the volume fraction of support material should be minimized and/or it should have high transparency to the activating photons. To enhance volumetric illumination, in an embodiment which employs a substrate, the substrate material is preferably made from glass or other materials transparent or semitransparent to the photoactivating wavelengths. This is possible using three dimensionally reticulated photocatalytic semiconductor unit. In an embodiment which bonds or chemically integrates the substrate with the semiconductor, the unit is also preferably made from transparent or semitransparent materials.
The water flow through the catalyst should be turbulent to improve mixing and mass transfer rates between the organic contaminants and the oxidizing species generated at the catalyst surface. Laminar flow should be avoided.
The reticulated structure utilized in the present invention substantially represents a breakthrough over the prior art and allows for the commercial use of photocatalytic technology in ultrapure water production because it optimizes mass transfer, surface area, illumination, water flow, durability, rigidity, etc. The photocatalytic semiconductor unit provides a high surface area, rigid structure to which the photocatalyst is adhered or incorporate. The interstitial struts forming the open celled structure of the photocatalytic semiconductor unit are relatively thin, so volume fraction of substrate support material is low and flow is not significantly restricted. The ramification and alignment of the struts with respect to the flow direction will generate tortuous flow paths and enhanced mass transport. The rigidity of the support structure provides a stable base to permanently attach or incorporate a highly active TiO2 surface.